Electric compressor or pump motor with rolling rotor



March 1, 1955 S. J. STEENSEN ELECTRIC COMPRESSOR OR PUMP MOTOR WITH ROLLING ROTOR Filed July 2, 1952 March l, 1955 s. J. sTEENsEN 2,703,370

ELECTRIC COMPRESSOR OR PUMP MOTOR WITH ROLLING ROTOR Filed July 2, 1952 4 Sheets-Sheet 2 March l, 1955 s. J. sTEENsEN ELECTRIC COMPRESSOR 0R PUMP MOTOR WITH ROLLING ROTOR 4 Sheets--Sheet 3 Filed July 2, 1952 March l, 1955 s. J; sTEENsEN 2,703,370 ELECTRIC COMPRESSOR OR PUMP MOTOR WITH ROLLING ROTOR 4 Sheets-Sheet 4 Filed July 2, 1952 United States Patent O ELECTRIC COMPRESSOR OR PUIVIP MOTOR WITH ROLLING ROTOR Sverre Johan Steensen, Ulleval Hageby, Oslo, Norway Application July 2, 1952, Serial No. 296,798

14 Claims. (Cl. S10-82) This invention relates to compressors or pumps of the type comprising a cylindrical piston rolling within a cylindrical pump housing, and having a magnetic driving means which applies the rolling movement to the piston through the wall of the pump housing. This magnetic drive avoids the necessity of providing a mechanical connection between the rolling piston and an outer driving means so that the compressor compartment may be completely sealed except for inlet and outlet ports, thus eliminating the necessity for stuing boxes which will become leaky and require periodic maintenance and replacement.

The invention is especially concerned with the magnetic driving means.

It has previously been known that a rolling movement may be applied to a piston by means of magnets coupled with the piston through the cylindrical wall of the pump housing, in which case said wall is usually relatively thin. Such coupling does not require any direct mechanical connection between the piston and its driving means. Annular magnets, rotating concentric with and externally of the cylindrical pump housing and driven by a separate motor have been suggested. Also a magnetic lield which is displaced stepwise along the circumference of the pump housing may be used, which eld can be obtained by means of tixed electromagnets which are switched in and out in turn by a rotating multicontact switch.

The main object of the invention is to improve driving means of the type using a number ot` xed electromagnets which are evenly distributed along the outer circumference of the cylindrical pump housing, and more specifically to avoid the necessity of using a rotating switch which has to be driven by a motor, or by a step by step magnet of the electromagnetic type.

This object is attained according to the invention, by dividing the coils of evenly distributed electromagnets into groups such that the resultant electromagnetic force of any group, when connected to an electric current source, is displaced up to 180 electric degrees relative to the resultant electromagnetic force of the adjacent group of coils.

Preferably, the rolling piston is magnetized in its axial direction, such as by means of an annular permanent magnet forming part of the piston.

These and other features of the invention will be readily understood from the description and. from the drawings, in which Fig. 1 shows a compressor in radial section along the broken line II--II in Fig. 2,

Fig. 2 is an axial section of the compressor shown in Fig. l, and

Fig. 3 illustrates diagrammatically the electrical connection of the electromagnet coils of the compressor shown in Figs. 1 and 2.

Figs. 4 and 5 show a modified construction of the compressor in radial and axial section respectively, Fig. 4 being a section along the line IV-IV of Fig. 5.

Fig. 6 illustrates an embodiment of the magnetic driving means especially suitable for alternating current, and

Fig. 7 illustrates diagrammatically the electrical connections for the embodiment shown in Fig. 6.

Figs. 8 and 9 show a further embodiment of a compressor in radial section and axial section respectively, Fig. 8 being taken along the line VIII-VIII of Fig. 9, and v Figs. 10 and ll show an embodiment of a double or twin compressor in radial and axial section respectively, Fig. 1l being taken along the line XI-XI of Fig. l0.

The compressor shown in Figs. l and 2, which is generally numbered 50, consists of an inner cylindrical piston 1 which may roll along the inner wall of a cylindrical pump housing 2. As will appear from Fig. 2, the piston is built up from an annular magnet 1 which is magnetized in its axial direction as indicated in the conventional way by letters N and S, and two disk shaped end pieces 1 of ferrous material which form pole shoes. These three parts, viz. the annular magnet and the two end pieces, are held together by a central bolt 8 the end portions 9 of which are of a somewhat reduced diameter. Furthermore, said piston has an axis parallel slot 5 which is radially directed and into ywhich is inserted a separating wall 4 which is freely moveable radially in said slot 5.

The pump housing consists of a thin metallic cylindrical sleeve 2 closed at both ends by end pieces 10 and 10 which are pushed into the cylinder 2 and which have collars 15 resting against the ends of said cylinder. After having been pushed into the cylinder the end pieces are secured in said position in a pressuretight manner, such as by soldering or welding. The piston 1 is of such dimensions that it will have sliding lit between the endpieces 10 and 10', and some clearance within the cylindrical pump housing, so that it may roll along said cylindrical wall without moving axially. In the position shown in Fig. l the piston is in contact with the upper portion of the pump housing.

The space formed between the end pieces 10 and 10',

' the inner Wall of the pump housing and the outer surface of the piston is divided into two compartments 6 and 7 by the separating wall 4. One of these compartments is the suction compartment and the other the pressure compartment, dependent upon the rolling direction of the piston. Thus, when the piston is rolling clockwise as seen in Fig. l, 7 will be the suction compartment and 6 the pressure compartment. Said compartments communicate with annular channels 11 and 12 in the end pieces 10 and 10 respectively through borings 11 and 12 respectively in the piston. The annular channels 11 and 12' connect with inlet and outlet ports 11 and 12 respectively in the two end pieces 10 and 10 respectively.

As appears from Figs. l and 2, the end pieces 10 and 10' have centrally arranged pins 3 which are directed into the pump housing and located within central bores 3 formed in the pole pieces 10 and 10', clearance being provided so that the piston 1 may rotate epicycloidally around the pins.

The rolling piston is driven magnetically by means ot' four electromagnets the cores 16, 17, 18 and 19 of which are evenly distributed along the outer circumference of the pump housing 2. As may be seen in Fig. 2, said cores have the form of horse shoe magnets and are preferably formed of laminated iron. When magnetized by their respective coils, said magnetic cores act upon the permanently magnetized piston 1 through the thin cylindrical wall 2 and the air gaps represented by the compartments 6 and 7, as may be seen in Figs. l and 2. The

opposite magnet cores 16 and 18 are each provided with two coils, 20 and 25 being associated with core 16, and 23 and 22 being associated with core 18. Each of the other two magnets 17 and 19 have only one coil, 21 being associated with core 17 and 24 being associated with core 19.

Said coils 20, 21, 22, 23, 24, and 25 are divided into two groups, the coils 20, 21 and 22 belonging to the first group and the coils 23, 24, and 25 to the second group. Said two groups of coils are switched in and out alternately by appropriate means to be described below.

important feature of the invention resides in the choice of the number of turns in the three coils belonging to a group in such way that they will provide a different number of ampere-turns, increasing in the rolling direction of the piston. For instance, the three coils forming the rst group, 20, 21 and 22, may have 100, 200 and 300 ampere-turns respectively. Thus, the resultant force of 0 this group of magnet coils Will not be located along the axis of the core 17 as would be the case it the coils con Patented Mar. 1, 1955 3 tained an equal number of ampere-turns, but is displace in, the rolling direction of the piston relative to-the axis of core 17. Thus, provided the above mentioned number of ampere-turns be used, for the iirst group thc resultant of the magnetic force will be located atl the corner bet tween magnets 17 and 18. Correspondingly, for thc second group, if coils 23, 24 and 25 have 100, 200, and 300 ampere-turns respectively, the resultant will be at the corner between magnets 17 and 19.

Assuming the right or iirst coil group, i. e. coils 20, 21 and 22, is supplied with current when the piston 1 is in the position shown in Fig. 1 the piston will be forced to roll alongthe right inner wall of the pump housing. i'. e. clockwise towards the position which corresponds to the resultant magnetic force of said group of coils.

During the first interval of the movement; tlie weaker` magnet 17 will become more important than the stronger magnet, 13, which is due to the fact that the air gap between the piston and magnet 17 is less than the air gap between the piston and magnet 1%. However, during thc rolling movement of the piston the air gap will be gradually` reduced so that the piston will continue to roll until it has reached a 180 position, which is the position oppositeto that ofy Fig. 1. ln this latter position the lower part of. the outer surface of the piston 1 will be in contactwith the periphery of the pump housing 2 and the separatingpwall 4 will be forced into slot Si During this part of the movement the liquid or the gas which may be presentin compartment 6, will be pressed out through ythe channels 11 and 11 and the outlet 11". time compartment 7 will increase in volume and cause liquid or gas to be drawn into the same through inlet 12' and channels 12 and 12.

The separating wall lmay be held against the inner wall of the pump housing 2 by means of some known means, such as a spring placed in the bottom of the slot- 5, o ry by magnetic action, said separating wall 4in such case` being made from ferro-magnetic material.

When the piston 1 reaches its lowermostposition the current supply to the iirst coil group is interrupted. Simuh taneously the second group, comprising the coils 23, 24 and 25, is switched in so that the piston will continue its rolling movement.

Owing to the fact that the piston 1 has a smaller diameter than the inner wall of the pump housing 2, during operation the separating wall t will be displaced slowly along the inner circumference of the pump housing op posite to the movement of the piston. rl`his has been taken into consideration by forming both communicating channels 11 and 12 as annular channels, whereby intake and exhaust may be accomplished irrespective of the position of the separating wall 4.

The periodic switching in and out of the coil groups may be carried out in different ways. Fory instance, both circuits may be controlled by an inverter of the pendulum type or the like. a direct current source being used. ln Fig. 3 is shown diagrammaticaliy how theconipressor may be driven by an alternating current source. Each of the coil groups are connected in series with vacuum tubes, such as diode rectiers 26 and 27 respectively, the series circuits being connected in parallel with analternating current supply with the diodes arranged to conduct in opposite directions. The magnet cores have also been illustrated diagranimatically so as to show that the coils 22 and 23 as well as coils 20 and 25 are arranged on common cores respectively.

When, as in the present case. only two groups .of magnet coils are used, the magnetic resultant vector will be displaced 180 geometrical or electrical degrees when the subsequent group is switched in, and, of the, system bel supplied with 50 cycle alternating current, the speed of the'compressor piston will be 50 revolutions per second or 3000 revolutions per minute. lf, for instance, four coil groups are used, the displacement of the magnetic. resultant vector will still be 180 electrical degrees, but will be only 90 geometrical degrees. In this latter case the piston will rotate at half speed as compared with the two coil group embodiment, i. e. 1500 revolutions per minute. Obviously any even number of groups may be used. giving a speed which is equal to 3000 multiplied by 2/11, in which n is the numb-er of coil groups used.

Figs. 4 and 5 show the same driving system described above used on a modified construction of compressor, generally numbered 60. The rolling piston 1 consists of a thickwalled hollow cylinder which may roll on a fixed At the same solid core 29, 29. Said hollow cylinder 1 is arranged within av cylindrical housing consisting'of a thin` walled hollow cylinder 2 closed at each end by end pieces 10 in the same way, as the pump housing shown in Figs. 1 and 2. Whereas the hollow cylindrical piston 1 has direct contact with the core 29, 29 it does not, at any point, touch the cylindrical wall 2, a very narrow space being present which is not visible in the drawing. The two chambers separatedby the separating wall 4, are numbered 6 and '7, 6 being the pressure chamber and 7 being the suction chamber for clockwise rotation. Said separating wall 4 is inserted into a slot in the core 29, 29', which core is fixed, by an axial bolt 8, to both end pieces and prevented from being turned about the bolt 3. The space between the rolling piston 1 and the cylindric housing 2 is numbered 28 and does not form part of the compressor compartment.

The compressor as shown in Figs. 4 and 5 acts in the same way as that described above with reference to Figs. l and 2.

However, the modiiied'construction has the advantage that the core 29, 29 as Well'as the hollow cylindric piston 1 may be permanently magnetized, as by annular magnets 29 and 1 respectively, each having annular pole shoes 29 and 1 respectively. Thisarrangement will give a more responsive action as comparedwith the rst embodiment and a better contact between the fixed core 29, 29 and the rolling piston 1. The drivingY means for the compressor according to Figs. 4 and'5 is as previously described in connection with Figs. l, 2 and 3, and the operation is the same.

In Fig. 6 is shown a driving arrangement especially designated for alternating current. Six magnets 30,' 32, 34, 36, 38 and 40 are evenly distributed along the outer circumference of the pump housing and divided into two groups each comprising three magnets, 30, 32, 34 and 36, 38, 40 respectively. Each of said magnets is provided with one magnetizing coil 31, 33, 35,' 37, 39 and `41 respectively. The compressor itself may be of the same type as described above in connection with Figs. 4 and 5, and is therefore numbered 60, but it should be understood that a compressor as described in connection with Figs. 1 and 2 maybe used with the driving system shown in Fig. 6.

The three coils of each group are arranged so that they will give a resultant magnetization in the same direction. The coiling direction of all the six coils is the same, which means that all willV cause the same direc- -tion of magnetization for the same current direction.

lcoils will act simultaneously. Owing to the fact that the -`*piston is polarized, the force which acts on the piston will be doubled as compared with the driving embodiment according to Figs. 1, 2, 4 and 5, one magnet group. pushing the lpiston away whereas the other group -attracts the saine.

ynecessaryto use a more powerful magnetic system which will give an increased pressure as compared with inagnetic systems of the aforesaid embodiments.

Such need for an increased pressure may be met when the driving means and the compressor are separated in some way. Two such embodiments are now to be described with reference to Figs. 8, 9 and 10,v 11, of which Fig. 8 is asection taken along line VIII-VIII in Fig. 9 and Fig. 11 is a section taken along line XI-XI in Fig. 10.

In Fig. 8 the compressor, generally numberedy 70, may be of any type having a rolling piston. Said piston 1 is fixed to an axial bolt 8 and arranged within a compartment deiined by the inner surface of an open-ended cylinder 42 having collars or flanges 42', and two'thinwalledendcaps. 2". The inner wall of the cylinder 42 has a smaller diameter than the inner walls of the cylindric portion 2 of the endcaps 2, the latter diameter being the same as the outer diameter of the collars 42'.

By means of the axial bolt 8 and nuts or rings 9 on the reduced ends 9 of said bolt 8, two solid cylindrical iron bodies 1 and 1" are fixed to the ends of the piston 1. Said nuts or rings 9' are arranged in recesses in one end portion of said iron'bodies 1' and 1" respectively, as shown. The diameter of said bodies is less than the inner diameter of the cylindric portions 2 of the end caps 2 so as to form annular gaps 43 therebetween. The end bodies 1 and 1act as pole shoes for the piston 1 which forms a permanent magnet, as indicated in the conventional way by letters N and S. `As will be understood said piston 1, pole shoes 1 and 1" and bolt 8 form together a rigid system which will oscillate within the closed compressor compartment by means of magnetic forces applied thereto.

Due to the fact that there is a close fit between the ends of the open-ended cylinder 42 and the inner end surfaces of the pole shoes 1' and 1", said end surfaces and said cylinder 42 define the compressor compartment.

Outside the compressor housing formed by the hollow cylinder 42 and the end-caps 2', 2" are arranged six axis parallel electro-magnets consisting of laminated cores 30, 32, 34, 36, 38, and each having one magnetizing coil 31, 33, 35, 37, 39 and 41 respectively. As in the embodiments described above, said electro-magnets are evenly distributed along the outer circumference of the compressor housing.

As will be understood,` the electro-magnets and the pole shoes 1' and 1" may have any practical axial length corresponding to a compressor compartment of a given size, so as to give a driving force of suiiicient magnitude.

The pressure and suction compartments of the compressors are numbered 6 and 7 respectively, and are separated by a separating wall 4, which is of the same length as the compressor compartment proper, and which is movable in a radial direction in a slot 45 in the wall of the cylinder 42. Said separating wall 4 is urged against the outer surface of the piston 1 by springs 46 arranged within sockets 44 and 44 which are inserted in a gas tight manner in the cylinder 42, as shown in Figs. 8 and 9. The outer portion of said sockets 44 and 44 have internal threads for nipples 47 which nipples have an outer ange and two threaded portions, as well as a bore 48 as illustrated in Fig. 8. The inner end walls of said sockets 44 and 44 have slots 45 which are inline with the beforementioned slot in the wall of the cylinder 42, as well as openings 49 and 49 respectively. Said last mentioned openings communicate with the pressure and suction compartments 6 and 7 respectively.

Between the portions of the outer edge of the separating wall 4 located within the sockets, and the inner ends of the nipples 47, are placed coil springs 46, the tension of which may be adjusted by screwing the nipples more or less into the sockets 44 and 44. The outer end of each of the nipples 47 is used to provide the outer connections to the pressure and suction compartment respectively.

In order that the piston 1 shall always have good contact with the inner wall of the hollow cylinder 42, the air-gaps 43 between the pole shoes 1' and 1 and the inner wall of the cylindric portions of end caps 2 are made slightly greater than the gaps between the piston 1 and the cylinder 42 forming the suction and pressure compartments 7 and 6. Thereby is obtained the additional advantage that the pole shoes will not stick to the inner walls 2.

The stationary part of the driving means, consisting of six electro-magnets evenly distributed along the outer circumference of the compressor housing, is also in this case divided into two or three groups dependent upon whether direct current and inverter, single phase or three phase alternating current is used.

ln the embodiment according to Figs. l0 and l1 an inverse principle is used as compared with the embodiment just described. In this case the driving means is arranged in the middle, and two compressors on either side of said driving means. The compressors are generally numbered 80 and 80', and are of exactly the same size and capacity, so that it will only be necessary to describe one of them. The rolling piston 51 is placed within the cylindrical compressor compartment defined by a thick-walled ring member 52 and two cylindric end walls 87 and 88 clamped to said ring 52 by throughgoing screws 86, as shown in the drawing. The separating wall 54 is inserted in an axis parallel slot in the ring 52, and is urged v that the pistons 51 and 51 magnetizing coils 65, 67, 69, 72, 74

ture 94 which is laminated, as shown, and arranged within a compartment defined by a thin-walled sleeve closed at both ends in gas-tight manner by the inner cylindric end walls 88 and 88 belonging to the compressors 80 and 80 respectively, as shown in Fig. ll.

In this way the middle compartment comprising the armature 94, as well as the two compressor compartments, are completely sealed but may communicate with one another. However, there will be no communication between said compartments, which is due to the fact have a sliding iit between their respective end walls.

In the present case the stationary part of the driving means consists of a laminated stator 63 formed with inwardly directed cores 64, 66, 68, 71, 73 and 75 carrying and 76 respectively and intermediate portions 77, 78, 79, 81, 82 and 83.

`The armature 94 has good clearance between the endwalls 88 and 8S', and the difference between the inner diameter of the sleeve 96 and the outer diameter of said armature 94 is slightly larger than the difference between the inner diameter of the rings 52 and 52 and the outer diameter of the respective pistons 51 and 51', so as to ensure that the pistons will always be in good contact with the inner wall of the respective compressor compartments, and to prevent sticking of the armature. The

gap between the armature 94 and the sleeve 90 is nurnbered 91. Corresponding gaps are present between the bolt 53 and the central bores on the inner end walls 88 and 88', which gaps are numbered 89 and 89' respectively.

'The compressors have inlet and outlet openings 55 and 57 respectively, formed by bores in the rings 52 and 52', which openings communicate with inlet and outlet ports 56 and 58 respectively leading into the pressure and suction compartments 84 and 85 respectively, near the separating wall 54. Said bores S5 and 57 are preferably threaded so as to be connectable, in a gas-tight manner, with tubes, as will be obvious to those skilled in the art.

The driving means operates substantially in the same manner as previously described, the coils 65, 67, 69, 72, 74 and 76 being divided into groups and connected with a current supply as previously described. In the present case, however, it is not possible to use a polarised armature with the advantage of pulling as well as pushing forces. Therefore, the groups of coils can not be supplied with current simultaneously but must be switched in and out in sequence.

It is evident that the invention is not restricted to the embodiments shown and described above. So the number of magnets may be increased and divided into further groups. For instance, if four groups are used, as already mentioned, the compressor will rotate with half speed as compared with the compressors described above. lt is also obvious that the arrangement according to Fig. 6 may be used in connection with direct current and an inverter. ln the same way the arrangement according to Fig. 6 may also be used in connection with a threephase alternating current supply, the six coils being divided into three groups and each group connected to one phase. In this way a rotating field is established which will cause the piston to roll in the desired direction. In connection with three-phase supply the number of magnets may be increased, for instance to nine, with three magnets per group and phase.

.l claim:

l. A motor comprising a cylindrical rotor arranged to rotate epicycloidally within a cylindrical housing, said rotor including a centrally disposed permanent magnet arranged to polarize the rotor in an axial direction, and magnetic driving means including a plurality of electromagnets distributed around the outer periphery of the cylindrical housing, said electromagnets cooperating with the permanently magnetized rotor to produce the epicycloidal rotation.

2. A motor lcomprising a cylindrical rotor arranged to rotate epicycloidally within afcylindrical housing, said rotor including a centrally disposed permanent magnet and a pair of cylindrical, ferro-magnetic pole shoes concentrically disposed at the poles ot said permanent -magnet, means securing said magnet and pole shoes as a unit for epicycloidal rotation within the housing, and magnetic driving means including a plurality of electromagnets distributed around the outer periphery of the cylindrical housing, said electromagnets cooperating with the permanently magnetized rotor -to produce the epicycloidal rotation.

3. Afmotor comprising a'cylindrical rotor arranged to rotate epicycloidally within a cylindrical housing, said rotor including a centrally disposed permanent magnet and a pair of cylindrical, ferro-magnetic pole shoes concentrically disposed at the polesof said permanent magnet and having their outer peripheries extending axially, means securing said magnet and pole shoes as a unit for lepicycloidal rotation within the housing, and magnetic driving means including a plurality of electromagnets distributed around the outer periphery of the cylindrical housing, the cores of said electromagnets being of horseshoe conguration with the poles of said horseshoes in contact with the outer periphery of the housing in juxtaposed relationship tothe periphery of the rotor pole shoes, said electromagnets cooperating with the permanently magnetized rotor to produce the epicycloidal rotation.

4. .A motor-comprising a cylindrical rotor arranged to rotate epicycioidally within a cylindrical housing, said rotor being magnetized by a centrally disposed permanent magnet arranged to polarize the rotor in an axial direction, and magnetic driving means including a plurality of electromagnets evenly distributed around the outer periphery ofthe cylindrical housing, the coils of said electromagnets being divided into groups so that when one of said groups is energized by an electric current the resultant magnetic vector of the group is positioned 1.8() electrical degrees from the resultant magnetic vector ot an adjacent group, the electromagnets cooperating with the permanently magnetized rotor to produce the epicycloidal rotation.

5. A rotor comprising a cylindrical rotor arranged to rotate epicycloidally within a cylindrical housing, said rotor including a centrally disposed permanent magnet and a pair of cylindrical, ferro-magnetic pole shoes concentrically disposed at the poles of said permanent magnet, means securing said magnet and pole shoes as a unit for epicycloidal'rotation within the housing, and magnetic driving means including a plurality of electromagnets evenly distributed around the outer periphery of the cylindricalhousing, the coils of said electromagnets being divided into groups so that when one of said groups is energized by an electric current the resultant magnetic vector of the group is positioned 18() electrical degrees from the resultant magnetic vector of an adjacent group, the electromagnets cooperating with the .permanently magnetized rotor to produce the epicycloidal rotation.

6. A motor according to claim 4 in which the number of ampere-turns in the magnetizing coils belonging to any group l-of electromagnets increases from coil to coil in the rolling direction of the rotor'.

7. A motor according to claim 4 in which four electromagnets'are evenl-y distributed around the outer periphery of the housing, two of the opposite electromaguets having two coils each while the remaining `two opposite electromagnets have one coil each.

8. A 'motor according 'to claim 4 in which there are two coil groupsyeach of the coils of a group being connccted in series with eachother and with a diode rectifier, Athe two series circuits thus formed `being connected in parallel with a source of single-phase alternating currentl with the series circuits oppositely polarized whereby the series circuits Ywill `be 'alternately energized by a current owing in opposite directions.

9. A motor comprising a cylindrical rotor arranged `to rotate epicycloidallywithin a cylindrical housing, said rotor being a hollow cylinder open at both ends, a stationary, cylindrical inner `core coaxial with the housing, said core having an outerperiphery about which the rotor rotates epicycloidally, the inner diameter of the rotor being greater than the diameter of `the core whereby a chamber is `formed`between the outer periphery of the core and the inner-periphery of the rotor, and magnetic driving means including a plurality of electromagnets distributed around vthe outer periphery of the cylindrical housing.

10. A motor according to claim 9 `in which the rotor includes a centrally disposed permanent magnet arranged topola'rize the rotor in an axial direction.

l1. A motor according to claim 9 in which the inner core includes a centrally disposed permanent magnet arranged to polarize the inner core in an axial direction.

12. A `motor according to claim 9 in which the rotor includes a centrally disposed permanent magnet arranged to polarize the rotor in an axial direction and the inner core includes a centrally disposed permanent magnet arranged to `polarize the inner core in an axial direction, the magnets having reverse polarity whereby the rotor and core are oppositely magnetized.

13. A motor according to claim 4 in which there are two groups of coils, each of said coils being Wound in the same direction, the coils of each group being connected in parallel with each other and with a source of single-phase alternating current, the polarity of the two groups being reversed -whereby the resultant magnetization of the two groups is reversed.

14. A motor according to claim 4 in which two groups of coils are connected to a common source of single phase alternating current, the coiling direction of each of said coils being chosen so that the resultant magnetization of one group of coils is opposite to the resultant magnetization of theother group.

References Cited in the tile of this patent UNITED STATES PATENTS 1,486,965 Janney Mar. 18, 1924 2,561,890 Stoddard July 24, 1951 2,579,865 Roters Dec. 25, 1951 

